1. Field of the Invention
The present invention relates, in general, to short-pulse fiber amplifiers, and more particularly to a chirped pulse fiber amplifier which exploits large nonlinear phase shifts to produce shorter pulses. This avoids the pulse-broadening that generally occurs owing to the gain bandwidth of the amplifier.
2. Description of the Background Art
There is rapidly growing interest in the development of efficient, compact, and stable ultrafast lasers for a variety of applications from the study of fundamental ultrafast processes in nature to precision machining. Fiber lasers offer a number of practical advantages over bulk solid-state lasers, including compact size, better thermal stability, freedom from misalignment, and lower cost. On the other hand, the pulse energy from fiber sources has not been comparable to that of solid-state devices.
Nonlinearity generally limits the energy of ultrashort pulses. This limitation is particularly severe in fiber devices owing to the small core and long interaction lengths. Excessive self-phase modulation (SPM) leads to pulse distortions and eventually the pulse may break up. Scaling of fiber amplifiers to the microjoule- and millijoule-pulse energies will require creative solutions for nonlinearity management. Self-similar amplification is one way to control nonlinearity. However, gain-bandwidth limitations eventually disturb the monotonic chirp, and thus limit the pulse energy, to the microjoule level thus far.
A key component of the design of high-energy fiber devices is to increase the mode diameter using multimode or photonic-crystal fibers. This allows a 30-50 times increase in the pulse energy. However, there are practical and fundamental limits to the size of the lowest-order transverse mode, which is required for high beam quality. Increased mode size implies a trade-off in numerical aperture, sensitivity to alignment and bend loss.
For the highest energies, chirped-pulse amplification (CPA) is required, along with a large mode area. In CPA, a pulse is stretched to reduce the detrimental nonlinear effects that can occur in the gain medium. After amplification, the pulse is dechirped, ideally to the duration of the initial pulse. The stretching is typically accomplished by dispersively broadening the pulse in a segment of fiber or with a diffraction-grating pair. For pulse energies of microjoules or greater, the dechirping is done with gratings, to avoid nonlinear effects in the presence of anomalous group-velocity dispersion (GVD), which are particularly limiting. In most prior work, CPA systems were designed with matched stretcher and compressor dispersions, and operated with minimum nonlinear phase shift (ΦNL) accumulated by the pulse. For ΦNL>1, the pulse duration and fidelity degrade. In other words, at low energy, the process of stretching and compression can thus be perfect. At higher energy, some nonlinear phase will be accumulated and this will degrade the temporal fidelity of the amplified pulse.
The total dispersion of a fiber stretcher differs from that of a grating pair, and this mismatch results in uncompensated third-order dispersion (TOD), which will distort and broaden the pulse, at least in linear propagation. At wavelengths where the fiber has normal GVD (such as 1 μm), the TOD of the fiber adds to that of the grating pair. Stretching ratios of thousands are used in CPA systems designed to generate microjoule and millijoule-energy pulses, in which case the effects of TOD would limit the dechirped pulse duration to the picosecond range. It has thus become “conventional wisdom” that fiber stretchers are unacceptable in CPA systems and, as a consequence, grating stretchers have become ubiquitous in these devices.
Published International Application No. WO 2006/113,507 to Wise et al. (hereinafter “Wise et al.”), which was published on Oct. 26, 2006, discloses a fiber CPA system that contradicts the prior conventional wisdom. In Wise et al., high pulse energies and peak powers can be obtained from fiber amplifiers, when the pulse is allowed to accumulate a nonlinear phase shift ΦNL which can compensate the third-order dispersion (TOD) in a fiber amplifier. More particularly, Wise et al. disclose a CPA system that employs a pulse stretcher and a pulse compressor which have dispersion characteristics that are mismatched to one another and thereby cause introduction of TOD during operation. While TOD would normally cause the amplified pulse to broaden to unacceptably-long duration, Wise et al. discovered that the TOD can be compensated by a nonlinear phase shift introduced into the system by either the amplifier or a dispersive fiber. The ratio of the nonlinear phase shift to the TOD is selected to reduce and preferably minimize the output pulse width of the compressor, which increases and preferably maximizes the peak power in the pulse.